As a light source for projection-type image display devices such as a liquid crystal projector, a light source of nearly a point light source and having high brightness and high color rendering property, for example, a high-pressure mercury lamp, has been used widely.
FIG. 15 is a front cross-sectional view showing a configuration of an arc tube 101 in a typical short arc high-pressure mercury lamp. A vessel of an arc tube 101 is made of quartz glass. The arc tube 101 includes a spheroidal light-emitting portion 102 in the central portion thereof, and columnar sealing portions 103 and 104 that are connected respectively with both sides of the light-emitting portion 102 and extend outward. Inside the light-emitting portion 102, a pair of electrodes 105 and 106 made of tungsten (W) are disposed opposite to each other. At rear ends of electrode bars 105a and 106a (circular in cross section) constituting a part of the electrodes 105 and 106, molybdenum (Mo) metal-foils 107 and 108 in rectangular strip shapes are bonded respectively by welding.
In the sealing portions 103 and 104, parts of the rear end sides of the electrode bars 105a and 106a are embedded. Although the electrode bars 105a and 106a are embedded therein, this does not mean that, in the portions of the electrode bars 105a and 106a located in the sealing portions 103 and 104, the entire outer peripheral surfaces thereof are perfectly in close contact with the quartz glass. That is, when a certain region of the outer peripheral surface of the electrode bars 105a and 106a is seen, unavoidably, a part of the outer peripheral surface is not in close contact with the quartz glass, while the remaining part is in close contact with the quartz glass. Thus, a minute gap is formed that allows the ingress of, for example, gas filled in the light-emitting portion 102. Especially in a region A where the electrode bar 105a (106a) and the metal-foil 107 (108) overlap one another, a gap X slightly larger than the above-described gap is formed as shown in an enlarged partial view of FIG. 15.
To cope with this, in general, the metal-foils 107 and 108 thinned to a thickness of 20 μm are used in the sealing portions 103 and 104, whereby the occurrence of the above-described gap during the sealing process can be suppressed, and the airtightness in the sealing portions 103 and 104 are secured. Further, the use of the thinned metal-foils 107 and 108 can relieve the stress caused by the difference in thermal expansion coefficient between the metal-foils 107, 108 and the quartz glass that is a constituent material of the sealing portions 103, 104. Thus, the occurrence of microcracks in that region can be suppressed.
However, regarding the electrode bars 105a and 106a, unlike the metal-foils 107 and 108, the stress caused by the difference in thermal expansion coefficient from the quartz glass during the sealing process cannot be relieved, and this sometimes causes microcracks in that region. Although only microcracks occur in this situation, in a high-pressure discharge lamp aimed at improving brightness by increasing the amount of filled mercury (e.g. 0.15 mg/mm3 or more) to increase a vapor pressure at the time of lighting, the following problem arises: starting from a few microcracks, microcracks gradually grow due to the stress applied by a high vapor pressure at the time of lighting, which has sometimes resulted in the fracture in the sealing portions 103 and 104 (for example, see Patent Document 1).
Especially in the regions where the gaps X are formed, i.e., the overlapped regions of the electrode bars 105a, 106a and the metal-foils 107, 108, cracks larger (e.g. twice or triple as large) than microcracks occurring in the other regions where the electrode bars 105a and 106a are embedded have occurred even before the lighting for unknown reasons.
In view of this, for suppressing this kind of fracture in the sealing portions 103 and 104, various techniques have been known conventionally. For example, in the case of an arc tube 201 shown in FIG. 16, single-layer coil members 202 and 203 are wrapped around the embedded portions of the electrode bars 105a and 106a, and embedded in the sealing portions 103 and 104, respectively. Thus, the stress applied to the sealing portions 103 and 104 caused by the difference in thermal expansion between the quartz glass and the electrode bars 105a and 106a can be relieved (see Patent Document 2).
Furthermore, for suppressing the occurrence and growth of the microcracks caused by the gap X shown in the enlarged cross-sectional view of the region A in FIG. 15, techniques to reduce or remove said gap X by improving the shape of the metal-foil 107 that is connected to the electrode bar 105a have been disclosed in Patent Documents 1, 3 to 5 and the like. Examples of the techniques include: (1) in the metal-foil 107, a portion connected to the electrode bar 105a is narrowed down; and (2) the narrowed metal-foil 107 is wrapped around a part of the outer peripheral surface of the electrode bar 105a.     Patent document 1: JP 3570414    Patent document 2: JP 11-176385 A    Patent document 3: JP 3518533    Patent document 4: JP 2004-265753 A    Patent document 5: JP 2004-296178 A